1. Technical Field of Invention
The present invention relates to a protection circuit for a bipolar power transistor. More particularly, the invention relates to a protection method and circuit for preventing a bipolar power transistor from leaving an area of safe operation.
2. Background of the Invention
It is known that the safe operative region of active devices is limited by the maximum power that can be dissipated thereby and by voltage and current values which must not be exceeded if one does not wish to damage the active device.
In the case of a power transistor, the limits of safe operation are set by the maximum allowed values in terms of temperature, power dissipation, and voltage and current levels of the power transistor.
Exceeding the temperature limits can cause melting of the materials, dissociations or generation of reaction products which alter the characteristics of the transistor, deterioration of the technical characteristics and of the materials, and increased surface currents.
Taking into account these limits in terms of current, voltage and power, the useful region in which a bipolar power transistor can operate is generally the one shown in FIG. 1, which is a chart in which the axis of the abscissa plots the collector voltage and the axis of the ordinate plots the collector current of a bipolar power transistor.
The chart of FIG. 1 plots various curves which subtend useful operating regions having different areas according to the pulse duration applied to the base terminal of the power transistor being considered.
Outside the indicated useful region, if the current or power limits are exceeded the transistor is typically damaged or destroyed.
The natural positive feedback of a bipolar power transistor with respect to temperature can become critical when working at voltages higher than the ones at which a first breakdown of the bipolar power transistor occurs, i.e., when the current capacity of the device quickly decreases as the applied voltage rises.
Without a carefully designed protection circuit, the operating point of the bipolar power transistor can leave the above-indicated useful region unless a power transistor is used which is oversized with respect to the working points used.
Conventional circuits for protecting bipolar power transistors suffer some drawbacks. In particular, the known protection circuit shown in FIG. 2 (showing a simplified manner which nonetheless shows its key aspects) in which the power transistor is designated by the reference numeral 1, uses two current sources 4 and 5 and bipolar transistors 2 and 3. The protection circuit of FIG. 2, however, is unable to precisely set the short-circuit current Isc of the associated power transistor 1 because current source 5 tends to reach saturation.
There are other existing circuit configurations which tend to eliminate the positive feedback problems of the bipolar power transistor with respect to the temperature. In these circuits, as the temperature increases, the circuits respond by forcing a lower Isc current. These circuits, however, cannot be applied in a context which requires the options of eliminating current consumption with a power-off command and setting a constant current Isc for higher voltages, i.e., for voltages up to the maximum voltage considered by the technology of the device.